1. Field of the Invention
This invention relates to deposition of films on a substrate by chemical vapor deposition and more particularly to deposition of films with improved properties by including exposure to ultraviolet light in the deposition process.
2. Description of Related Art
Advanced semiconductor devices having higher performance and greater functionality than previous devices are often characterized by decreasing device feature geometries. As device geometries become smaller, the dielectric constant of an insulating material used between conducting paths becomes an increasingly important factor in device performance. Reducing this value advantageously lowers power consumption, reduces crosstalk, and shortens signal delay for closely spaced conductors.
Organic polymers are advantageously used to provide insulating films for low dielectric constant applications. Many of these films are typically applied by chemical vapor deposition, a process that is widely used in the semiconductor industry. The dielectric constants of organic polymer films are typically between 1.5 and 3, considerably lower than the dielectric constant of approximately 4 of a silicon oxide (SiO.sub.2) film, the material often employed as an insulating material in conventional devices. However, while SiO.sub.2 adheres easily to the silicon, silicon-containing, or metal surfaces of typical semiconductor devices, organic polymer films do not generally adhere as well to these semiconductor substrates. In addition, organic polymer films often do not exhibit the hardness and thermal stability desired of films used as insulating layers in semiconductor devices.
It is known that exposing organic polymer films to ultraviolet (UV) radiation promotes cross-linking of polymers in the films, a process which is associated with increased hardness, improved thermal stability, improved film cohesion, and reduced subsequent outgassing of the films. For example, the improved thermal and mechanical stability obtained by cross-linking fluorocarbon polymer chains is described in R. A. Flinn et al. in "Engineering Materials and Their Applications," pp 370, 409 (2.sup.nd Ed., 1981). It is also known that simultaneous irradiation of organic materials during polymerization is often advantageous for promoting the completion of polymerization.
However, as stated previously, many low dielectric constant films are typically applied to semiconductor wafers in chemical vapor deposition (CVD) processes and conventional CVD systems are not designed to accommodate a light source directed at the wafer surface.
Thus, it would be desirable to provide a process to improve the hardness, thermal stability, and adhesion properties of low dielectric constant organic films. It would be desirable to improve these properties by cross-linking the polymer films by exposure to UV radiation. It would also be desirable to promote completion of polymerization during deposition by exposure to UV radiation. It would further be desirable to provide a CVD apparatus for the deposition of such films that includes a means to expose wafer surfaces to UV radiation.